The present invention is related to the optical system disclosed by R. M. Scott in U.S. Pat. No. 3,821,763, and to the projection system disclosed in the A. Offner U.S. Pat. No. 3,748,015 both of which are assigned to the Assignee of the instant application, and the disclosures of which are incorporated by reference herein. The Scott Patent discloses a restricted off-axis field optical system in which an annular slit is used to restrict the field to an annular zone centered on the optical axis, and the system is optically corrected to preferentially increase image quality in the annular zone. The Offner Patent discloses a catoptric, off-axis annular field optical system for forming in accurate micro detail an image of an object at unit magnification with high resolution including convex and concave mirrors in face-to-face relationship with their centers of curvature being nearly concentric. The mirrors are arranged to produce at least three reflections within the system, and they are used in the system where there are axial conjugates at unit magnification in planes normal to the axis of the system.
Another high performance optical system having a broad spectral band is disclosed in the A. Hoffner Pat. No. 4,293,186, assigned to the Assignee of the instant application and the disclosure of which is incorporated by reference herein. This patent discloses a restricted off-axis field optical system, which includes refractive elements, having means for obtaining stigmatic imagery in the restricted off-axis field over an extended spectral range by balancing the chromatic variation in focus at the center of the restricted off-axis field due to the variation of field curvature with color by introducing axial color aberration of the opposite sense. In one form of the Offner system, two meniscus elements or shells are employed, in which one shell functions to improve the monochromatic performance and the other shell functions to correct the variation of performance with wavelength. This is done by letting at least one of the shells depart from concentricity. However, the departure of the shells from concentricity introduces higher order aberrations, which are not significant at a speed of f/3, but which could cause difficulties at faster speeds.
As is known from the publication entitled: "Achievements in Optics", by A. Bouwers, Elsevier Publishing Company, Inc., 1946, (particularly pages 24, 25 and 39,) and the article appearing in the Journal of The Optical Society of America, Vol. 34, No. 5, May 1944, pp. 270 to 284, entitled: "New Catadioptric Meniscus System", by D. D. Maksutov, meniscus elements can be used to reduce or remove the spherical aberration of principal rays parallel to the optical axis. Methods of correcting the axial longitudinal color aberration of said such systems are also described in these references.
Other related patents and publications in this field include Russian Pat. No. 126,911, issued Mar. 30, 1959; French Pat. No. 784,063, issued July 22, 1935; U.S. Pat. No. 3,244,073, issued Apr. 5, 1966; U.S. Pat. No. 3,821,763, issued June 28, 1974; U.S. Pat. No. 3,951,546, issued Apr. 20, 1976 and U.S. Pat. No. 4,011,011, issued Mar. 8, 1977, which is a continuation of U.S. Patent Application Ser. No. 339,860, filed Mar. 9, 1973 (now abandoned); "Unit Magnification Optical System without Seidel Aberrations", by J. Dyson, Journal of The Optical Society of America, Vol. 49, p. 713, 1959. (discussed more fully hereinafter).
It is noted that throughout the following specification and claims, statements made referring to an annular field of mean radius H also refer to a restricted off-axis field at a distance H from the axis. The terms color or chromatic variation as used herein is intended in its extended sense, i.e., it includes variation with wavelength in any region and is not restricted to visible light.